When writing (a blog post, script, etc..) what is the proper way to indicate two or more instances of a single letter? For instance, in Monty Python's Bookshop Sketch:
C: I wonder if you might have a copy of "Rarnaby Budge"?
P: No, as I say, we're right out of Edmund Wells!
C: No, not Edmund Wells - Charles Dikkens.
P: (pause - eagerly) Charles Dickens??
P: (excitedly) You mean "Barnaby Rudge"!
C: No, "Rarnaby Budge" by Charles Dikkens. That's Dikkens with two Ks, the well-known Dutch author.
P: (slight pause) No, well we don't have "Rarnaby Budge" by Charles Dikkens with two Ks, the well-known Dutch author, and perhaps to save time I should add that we don't have "Karnaby Fudge" by Darles Chickens, or "Farmer of Sludge" by Marles Pickens, or even "Stickwick Stapers" by Farles Wickens with four M's and a silent Q!!!!! Why don't you try W. H. Smith's?
C: Ah did, They sent me here.
I had always believed that plural never uses an apostrophe before the 's' (it's only used for possession), but I have rarely seen in written material the format "four Ms". (On a side note, whoever wrote this transcript also used "two Ks".)
On a side note, and perhaps this should be a separate question, if a Compact Disc is a CD, then two Compact Discs would be two CDs right? (I see "CD's" written everywhere)